Briefly stated, a brushless motor is a motor in which the positions of magnetic poles of a rotor are detected by means of a detector directly coupled to the shaft of the rotor. In response to the detected position, semiconductor switching elements such as transistors, thyristors or the like are turn on and off so as to continuously generate torque in the motor. Field windings or a multi-segment permanent magnet are used for the rotor.
The torque is created by application of currents to stator or field windings in sequential order to produce a torque-inducing flux for moving a rotor. The DC currents are alternately switched about the field windings to create various current paths that produce magnetic flux orientations in a synchronized fashion. The magnetic flux so produced results in a torque on the motor that causes the desired rotational movement In order to ensure that current is applied to the proper motor phase, sensing devices are used to provide information about the position of the rotor. Typically, this information is derived through systems such as Hall sensors, optical sensors or resolvers. These different systems do not give an absolute position, but provide enough information to know the relative position of the rotor in one electrical period. Therefore, it is possible using these devices to energize the motor in such a way that it starts in every case in the correct direction.
Of the known position detection systems, the best known and most commonly used, especially in motors where economy and small size are of significant importance, are Hall sensors. However, the position of the Hall elements must be very precisely fixed. Further, the heat resisting temperature of a Hall element is limited, so that deterioration of the characteristics of the motor can occur if the motor is heavily loaded. Another problem with these sensing device is that they are prone to failure. Thus, the Hall devices significantly affect the overall reliability of the apparatus that incorporates them. Also, incorporating these sensing devices in the motor structure itself increases the motor size, cost, complexity, and power consumption. A number of wire leads must also be provided to each Hall effect device to bring out the information detected by the Hall device to a microprocessor or the like external to the motor shell.
A number of different solutions to indirect position detection which do not require sensors have been developed. For example, methods disclosed to date include: direct or indirect back EMF detection, as disclosed in V. D. Hair, "Direct Detection of Back EMF in Permanent Magnet Step Motors," Incremental Motion Control Systems and Devices Symposium, Urbana-Champaign, 1983, pp. 219-221, and K. M. King, "Stepping Motor Control", U.S. Pat. No. 4,136,308, Jan. 23, 1979; a current analysis, as disclosed in B. C. Kuo, A. Cassat, "On Current Detection in Variable-Reluctance Step Motors," Incremental Motion Control Systems and Devices, 6th Annual Symposium, Urbana-Champaign, 1977, pp. 205-220; and two third harmonic analyses, as disclosed in P. Ferraris, A. Vagati, F. Villata, "PM Brushless Motor: Self Commutating Prerogatives with Magnetically Anisotropic Rotor," Instituto di Elettriche, Politecnico di Torino, Italia, and R. Osseni, "Modelisation et Auto-Commutation des Moteurs Synchrones," EPFL No. 767, 1989. However, these methods do not provide any information about the position of the rotor at standstill. If the electrical drive system has been switched off and the rotor is not turning, it is not possible to know the rotor's actual position as related to the stator phases. Thus, at switch on, the motor may start in either the correct or incorrect direction. This may not matter for some applications, but in many applications, such as in driving the spindle motor in a disc drive, an incorrect starting direction is not acceptable.
One known effort to determine the starting position without the use of sensors is disclosed in U.S. Pat. No. 4,876,491. According to this method, a short current pulse is applied to each power phase of the motor, and the resulting motor current is measured to determine the positional information of the rotor based on the return pulse of greatest amplitude. However, the difference between the pulses returned from the different phases may be very small, and may be affected by temperature, noise, or differences between the phase inductances or phase resistances. Also, it is possible that the returned pulses may all be of equal amplitude and render a position measurement impossible.